A breath of compressed air

In brief:

• Untreated compressed air isn’t necessarily safe for breathing air purposes unless the air has been dried and atmospheric contaminants have been removed.
• Fixed-cycle dryers typically have a constant purge using significant volumes of compressed air and this incurs high operating costs.
• Dew point or moisture-loading dependent controls can save costs on main air dryers and purifiers.

Many industrial facilities need breathing air to protect workers from hazardous airborne contaminants. Often, general plant compressed air is used as a source. However, the conditions at the air compressor’s intake can’t be guaranteed. Purifiers are needed to clean and condition the air to ensure it meets required breathing air standards. Purifiers using fixed-cycle heatless desiccant dryers might use significant amounts of expensive compressed air for purging. This purge flow usually is higher than required because breathing air systems are subject to inlet flow rates that are less than the purifier rating point. Let’s explore the effects of purge flow in breathing air purifiers and discuss measures that reduce purge flow and save operating costs.

General

“It’s not usually a common or safe working practice to turn off the breathing air manually during work breaks to save purge flow.”

The environment around a paint booth or cleanroom might be clean, dry and free of vapors, but there’s no guarantee these same conditions exist at the intake of the compressor that provides breathing air. “We’ve been using breathing air filters to condition our air for our welding hoods”, says the welding supervisor at a highway coach manufacturer. “But we make vehicles, so, despite warning signs posted around our compressor rooms, very often idling vehicles cause our carbon monoxide alarms to ring regularly.” Contaminants that could affect the health of breathing air users include oil aerosols, oil vapors, solid particles, carbon monoxide and microorganisms.

Compressed air for breathing air applications must be purified properly to ensure that it meets the relevant local breathing air standards. Standards exist in most every country and dictate the protection required for workers exposed to hazardous environments. One example is the use of breathing hoods or masks. The compressed air sent to these devices must meet breathing air quality criteria, as determined by OSHA in the United States and CSA in Canada. These standards define acceptable levels of oxygen, nitrogen, carbon dioxide, carbon monoxide, moisture and other trace gases.

Figure 1. A typical breathing air purifier system contains a filtration system, a desiccant air dryer and a catalyst element. (Source: SPX)

Often desiccant-style breathing air purifiers are used to help condition the air to meet the required standards. These units (Figure 1) typically contain a filtration system, a desiccant air dryer and a catalyst element. The filtration system removes particles, liquid aerosols, oil vapors and odors that might be transmitted from the air compressor intake, through the distribution system to the worker. The catalyst converts carbon monoxide that might be present to more tolerable carbon dioxide. The desiccant dryer removes the water vapor so the catalyst can operate without contamination, as these are sensitive to the presence of water vapor.

Energy issues with purifiers

Fixed-cycle heatless desiccant dryers, the most common type of dryer used in breathing air purifiers, need significant volumes of air for internal purge. The dryers are designed to process the full rated compressed air flow at an inlet condition of 100° F compressed air temperature, 100 psig air pressure, saturated with water vapor and an ambient temperature of 100° F. These conditions require an average purge flow of about 15% of the dryer’s rated flow. For example, a dryer rated at 100 scfm would consume about 15 scfm of compressed air, equivalent to 4 hp of air compressor power. For purifiers using fixed-cycle dryers, this purge is constant and consumes significant volumes of expensive compressed air, often leading to unnecessarily high operating costs.

On average, purifiers are subject to much less than design conditions. Often these units are only partly loaded because they’ve been sized for future conditions of maximum production capacity. These factors often make a constant purge flow of 15% of the dryer rating unnecessary.

Breathing air consumption also is highly variable because of the nature of the work. For example, painting operations aren’t continuous. There are many changes in demand as personnel take breaks, set up the work or aren’t painting during evening and weekend shifts. It’s not usually a common or safe working practice to turn off the breathing air manually during work breaks to save purge flow, so, in most cases, the units are left to run 24 hours a day, seven days a week.

Real-life testing

A recent study highlighted a large highway coach manufacturer that installed breathing air purifiers to protect its paint booth workers. Flow meters were installed on sample units to measure inlet and outlet flows. This manufacturer was in a production slowdown and operating painting activities only 8 hr/day, but the purifiers ran continuously, including evenings and weekends. Measurements showed that the actual average breathing air loads were less than 5 scfm, or about 3% of the capacity of the installed purifiers. The purge flows averaged 35 scfm per unit, which is more than 7 times the actual breathing air used, and 47 times the purge required by this reduced flow. There were six breathing air purifiers at the site, consuming a total of 315 scfm in purge flow to process an average of about 30 cfm of breathing air. This compressed air amounted to $45,000 per year in electricity costs.